题目

How and why Jack came to China _____ not known. When and where to build the new library _____ not been decided.     A. is; has           B. are; has                 C. is; have            D. are; have   答案：A两个引导词是一个抽象概念时，谓语动词用单数形式。 You could see the runners very well from ______ we stood. A. which B. where  C. that  D. when